In the industry of hydrocarbon oil and gas production it has been proposed to radially expand a tubular element extending in a wellbore formed into an earth formation. The tubular element can be, for example, a wellbore casing which is, after expansion thereof, cemented in the wellbore. In conventional wellbore drilling the wellbore is drilled and cased in sections whereby after drilling and casing each section, the wellbore is drilled deeper and a next casing section is lowered through the previous casing section. Thus, the next casing section necessarily has to be of smaller outer diameter than the inner diameter of the previous casing section. By radially expanding each casing section after installation thereof in the wellbore, it is achieved that the lower wellbore part still is of a sufficiently large diameter.
It has been proposed to expand each casing section by pulling, pushing or pumping a rigid expander through the casing section whereby the expander has an outer diameter larger than the inner diameter of the unexpanded casing. By virtue of the phenomenon that the inner diameter of the casing after expansion is slightly larger than the outer diameter of the expander (generally referred to as “surplus expansion”), the expander can be moved through expanded casing portions with some clearance. However, a problem of the known expander is that it is impossible to move the expander through unexpanded portions of the casing.
It has further been proposed to apply a collapsible expander which can be moved through the casing when in the collapsed position. One such collapsible expander is disclosed in U.S. Pat. No. 6,012,523, which expander is provided with hingeable segments (also termed fingers) which axially slide over a conically shaped body portion to form the final expanded cone. A drawback of this expander is that the hinges of the segments are subjected to high (friction) loads during the expansion process. Another drawback of the expander is that small clearances between the segments cause extrusion of the tubular element into such clearances thereby causing axial tracks on the inside of the expanded tube, which tracks form insufficiently expanded portions at the inner surface of the tubular element.
Accordingly there is a need for an improved expander which overcomes the aforementioned drawbacks.